What "exact age" means
Most people give their age as a single whole number — "I'm 34." That's your age in completed years, and it's fine for conversation. Your exact age is more precise: it's how many full years, then how many additional months, then how many additional days have passed since your birth date.
So someone born on 15 March 1991, checked on 10 September 2026, isn't just "35". They're 35 years, 5 months, and 26 days. Same person, more resolution.
The manual method
Write today's date above your birth date, lined up in three columns — day, month, year — and subtract each column, right to left.
Two things can go negative:
- Days go negative. Borrow one month. Add the number of days in the previous month (the month before today's month) to the day column, and subtract 1 from the month column.
- Months go negative. Borrow one year. Add 12 to the month column, and subtract 1 from the year column.
Do the day borrow first if you need both — it can push the month column negative, which you then fix with the year borrow.
A worked example
Birth date 15 March 1991. Today 10 September 2026.
| Column | Today | Birth | Result |
|---|---|---|---|
| Day | 10 | 15 | 10 − 15 = −5 → borrow |
| Month | 9 | 3 | adjusted below |
| Year | 2026 | 1991 | 2026 − 1991 = 35 |
Day borrow: the month before September is August, which has 31 days. Day becomes 10 + 31 − 15 = 26. Month drops by 1, from 9 to 8.
Month: 8 − 3 = 5. Not negative, so no year borrow needed.
Year: stays 35.
Exact age: 35 years, 5 months, 26 days.
Age in total days, hours, and seconds
A different question — not "how many years and months" but "how many days, total". For that you count every day from your birth date to today, leap days included. There's no clean shortcut by hand; the usual approach is to convert both dates to a running day count and subtract.
As rough anchors: 1 year ≈ 365.2425 days, so 35 years is roughly 12,783 days, about 306,800 hours, or about 1.1 billion seconds. The age calculator gives the exact totals and updates the seconds live.
The edge cases that trip people up
- Which month's length do you borrow? Always the month before today's month, not the month before your birth month. In the example above that's August (31), not February.
- Leap years. A year is a leap year if it's divisible by 4 — except century years, which must be divisible by 400. So 2000 was a leap year; 1900 and 2100 are not.
- Born on 29 February. In non-leap years there's no 29th, so most systems treat the birthday as 28 February (occasionally 1 March). A "leapling" born in 2000 has had about 6 actual birthdays by 2026 but is 26 years old.
- Time of day. Exact age to the second depends on your birth time and time zone. Date-only calculations assume midnight and ignore zones, which can shift a to-the-second age by up to a day.
- Off-by-one on the birthday itself. On your birthday you've completed another year, so day and month are 0 — not "1 month" or "365 days". Some tools get this boundary wrong.
Why forms sometimes disagree by a day
Two calculators can return ages that differ by a day for the same inputs. Usually it's one of three things: they disagree about whether the end date (today) is counted, they handle the month-borrow with a different month's length, or one is using your local time zone and the other UTC. For legal purposes — voting, drinking, retirement, pension eligibility — the rule is almost always completed years on the date in question, which is the whole-number part and rarely ambiguous.
What exact age is used for
- Legal thresholds — turning 18, 21, or a retirement age on a specific date.
- "Age on a past or future date" — how old you were at an event, or will be at one.
- Infant age — babies are tracked in weeks, then months, where the day count genuinely matters.
- Health inputs — age feeds formulas like TDEE and calorie targets, though those only need completed years.
Frequently asked questions
How do I calculate my age from my date of birth?
Subtract your birth date from today in three columns — day, month, year. If the day result is negative, add the previous month's length and drop the month by one. If the month result is then negative, add 12 and drop the year by one. What's left is your age in years, months, and days.
What is my exact age right now?
Your completed years is this year minus your birth year, minus one if your birthday hasn't happened yet this year. The months and days are the leftover gap since your last birthday. A calculator is the quickest way to get all three parts plus the total day count.
How many days old am I?
Count every calendar day from your birth date to today, including leap days. As a rough check, multiply your age in years by 365.2425. For an exact figure, use a calculator — the leap-day bookkeeping is where hand counts go wrong.
How is age calculated for someone born on 29 February?
They age one year every year like everyone else — a leapling born in 2004 is 22 in 2026. Their birthday only lands on an actual 29 February in leap years; in other years it's marked on 28 February or 1 March depending on the context.
Does age calculation count today?
For years, months, and days it doesn't matter — you're measuring the gap up to today. For "how many days old", conventions differ by a day depending on whether the end date is included. Most calculators count from birth date up to but not including today.